There are many situations in which it is desirable to sense the presence of and/or quantity of contaminants such as hydrocarbons or other non-volatile residue on surfaces. For example, satellites or other spacecraft contain many critical surfaces which must be kept strictly clean prior to launch to avoid degradation of optical, heat rejection, or other properties. In these situations, in addition to insuring cleanliness of the environment, it is also desirable to monitor directly the cleanliness of the surfaces. Large particles on the surfaces are detectable by optical scattering techniques. For detection and measurement of molecular contamination such as non-volatile residue, typically consisting of a hydrocarbon film such as an oil film, the detection is typically accomplished by mounting a "witness" plate or surface near the surface to be monitored. Periodically, this witness plate is removed and analyzed for contaminants which it and, presumably, the surface of interest, may have acquired. This analysis typically involves washing the witness plate with solvent and analyzing the solvent for contaminants. Alternatively, the surface may be analyzed for contaminants by spectrometry.
Contaminant surface deposition real-time monitoring may be accomplished with quartz crystal microbalances placed proximate the surface being monitored for the duration of the possible contaminant deposition. These quartz crystal devices, however, are sensitive to humidity fluctuations, and so are best suited for vacuum environments.
In another scheme for detecting the presence of surface contaminants, the surface is irradiated with ultraviolet light to ionize the contaminants, and an extremely sensitive surface current detection device is used to detect current flow from the ionization, which is related to the amount and type of contaminant. In such systems, however, the surface to be monitored must be measured prior to contaminant exposure, when it is absolutely clean, to provide the comparison current necessary for contaminant sensing.